Galvanic 3.pdf

8/20/2019 Galvanic 3.pdf

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Material Properties and Corrosion

Course No: T06-001

Credit: 6 PDH

A. Bhatia

Continuing Education and Development, Inc.9 Greyridge Farm CourtStony Point, NY 10980

P: (877) 322-5800F: (877) 322-4774

[email protected]


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Science and Reactor Fundamentals – Materials 1

CNSC Technical Training Group

Revision 1 – January 2003

Table of Contents

1.0 OBJECTIVES.......................................................................................... 3

2.0 MATERIAL PROPERTIES.................................................................... 5

2.1 Introduction .................................................................................... 5

2.2 Mechanical and Thermal Stress..................................................... 5

2.2.1 Mechanical Stress and Strain...................................................... 5

2.2.2 Thermal Stress and Strain........................................................... 6

2.2.3 Hoop Stress ................................................................................... 8

2.2.4 Residual Stress.............................................................................. 8

2.2.6 Consequences of Exceeding Stress Limits of Materials .......... 10

2.2.8 Why valves should be cracked open during warm-up of asystem ...................................................................................................... 11

2.3 Brittle/Ductile Transition ............................................................. 11

2.3.1

Ductility....................................................................................... 11 2.3.2 Brittleness.................................................................................... 11

2.3.3 Nil-Ductility Transition Temperature...................................... 12

2.3.4 Differences between Ductile and Brittle Fracture................... 13

2.3.5 Operating Limitations for Materials Exhibiting aDuctile/Brittle Transition Temperature............................................... 13

2.4 Creep.............................................................................................. 14

2.4.1 Why a large shaft becomes deformed when at rest ................. 14

2.4.2 Why rolling a large shaft prior to operation reduces thedeformation ............................................................................................ 15

2.5 Fatigue .......................................................................................... 15

2.5.1 Failure mechanisms due to fatigue caused by work hardening ...................................................................................................... 15

2.6 Erosion .......................................................................................... 15

2.6.1 Surface failure due to wear and erosion of materials ............. 15

2.6.2 Abrasion...................................................................................... 16

2.6.3 Adhesion...................................................................................... 17

2.6.4 Fretting........................................................................................ 17

Review Questions ................................................................................... 18

3.0 CORROSION......................................................................................... 20

3.1 Introduction .................................................................................. 20 3.2 Types of Corrosion........................................................................ 20

3.2.1 Uniform Corrosion..................................................................... 20

3.2.2 Galvanic Corrosion.................................................................... 21

3.2.3 Pitting and Crevice Corrosion .................................................. 23


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3.2.4 Stress Corrosion Cracking (SCC)............................................. 24

3.2.5 Erosion Corrosion......................................................................24

3.2.6 Microbiologically Induced Corrosion (MIC)........................... 24

3.3 Carbon Steel Based Systems...................................................... 25

3.3.1 pH Control in Carbon Steel Systems........................................ 25

3.3.2 The importance of dissolved oxygen control, and typicalmethods for proper control ................................................................... 28

3.4 The Importance of Conductivity Control, and Typical Methods

for Proper Control ................................................................................... 30

3.5 Stainless Steel Based Systems....................................................... 32

3.5.1 SCC in the Moderator................................................................ 32

3.5.2 SCC in the End-shield Cooling and the Liquid Zone ControlSystems.................................................................................................... 32

3.5.4 Conductivity Control in Stainless Steel Based Systems, andTypical Methods to Maintain Proper Control..................................... 35

3.6 Scale Formation............................................................................ 36 3.6.1 Mechanisms for Formation and Adverse Consequences ofScale Formation on Boiler Tubes-Methods Used to Minimize Scale36

3.6.2 Adverse Effects of Sludge and Scale Formation...................... 37


4.0 PRESSURE TUBES AND FUEL BUNDLES..................................... 40

4.1 Introduction .................................................................................. 40

4.2 Effects of Radiation on Common Materials ................................ 40

4.2.1 Oils and Greases......................................................................... 40

4.2.2 Plastics......................................................................................... 41

4.2.3 Metals .......................................................................................... 41

4.2.4 Carbon Steel ............................................................................... 42

4.2.5 Zirconium Alloys........................................................................ 42

4.2.6 Concrete ...................................................................................... 42

4.2.7 Factors Affecting Creep in Pressure Tubes ............................. 43

4.3 Hydrogen Embrittlement, Delayed Hydride Cracking, Blisteringof Pressure Tubes and Contributing Factors ........................................ 46

4.4 Minimizing Delayed Hydride Cracking During Unit Startup and

Cool-Down ............................................................................................... 52



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1.0 OBJECTIVES

This course covers the following areas pertaining to Mechanical

Properties:

! Mechanical and Thermal Stress! Corrosion

! Pressure Tubes and Fuel Bundles

At the completion of training the participant will be able to:

Mechanical and Thermal Stress

! define terms as they relate to materials: mechanical stress and

strain, hoop stress, thermal expansion, differential thermalexpansion, thermal shock & residual stress

! describe factors which cause mechanical and thermal stress in a

component! explain the consequences of exceeding stress limits in materials

! explain why heating and cool down rates are limited

! define the following properties of materials: ductility, brittleness &

nil-ductility transition

! explain the differences between ductile and brittle fracture

! explain why a material exhibiting a ductile/brittle transition

temperature has operating limitations with respect to temperature

! define creep as it relates to materials

! explain why a large shaft becomes deformed when at rest

! explain why rolling a large shaft prior to operation reduces thedeformation

! describe fatigue failure and work hardening

Corrosion

! describe the erosion of material

! describe wear or surface failure of materials

! given a plant system and associated chemical parameters with theirnormal operating ranges, explain the consequences of operating

outside this range and the control methods used

! describe the following corrosion types: uniform, galvanic, pitting

and crevice, stress corrosion cracking, erosion & microbiologicallyinduced

! explain the importance of pH control in carbon steel basedsystems, including the significance of a magnetite layer, and

describe the typical methods used to maintain proper control


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! explain the importance of dissolved oxygen control in carbon steel based systems, and describe the typical methods used to maintain

proper control

! explain the importance of conductivity control, and describe the

typical methods used to maintain proper control

! define the term stress corrosion cracking (SCC), and state theconditions required to promote SCC

! explain the importance of pH control in stainless steel basedsystems, and describe the typical methods used to maintain propercontrol

! explain the importance of conductivity control, and describe thetypical methods used to maintain proper control

! explain how scale can be formed on boiler tubes, state the adverseconsequences of scale formation, and state the methods used to

minimize scale formation

Pressure Tubes and Fuel Bundles! state the effect of radiation on materials: oils and greases, plastics,

metals & concrete

! describe the causes of creep in pressure tubes

! describe the process of hydrogen embrittlement and the occurrence

of delayed hydride cracking and blistering of pressure tubes,including the factors affecting it

! explain how temperature cycling and reduced heat transport pressure can be used to minimize the potential for delayed hydride

cracking in pressure tubes during start-up and cool-down


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Notes:2.0 MATERIAL PROPERTIES

2.1 INTRODUCTION

Materials of many types are important both in the design and operation

of nuclear power plants. All materials may fail due to external orinternal forces, such as mechanical and thermal stress, or to associated

phenomena like creep, fatigue and erosion. In this module, we will

discuss the main causes and consequences of mechanical failure formaterials commonly encountered and over which an operator has some

control.

2.2 MECHANICAL AND THERMAL STRESS

Stress is a familiar word today, but the technical definitions of stress andits resulting effect, strain, are quite precise. The following are

definitions of these and associated terms.

2.2.1 Mechanical Stress and Strain

Whenever a load or force is applied to a material, that material is subject

to a stress defined as the force applied over a unit area. (This conceptallows us to ignore the size of the unit while exploring the effect of the

force on the physical state of the part.) For example, external force due

to gravity stresses all materials.

There are three types of stress:

! Tensile stress that tends to lengthen a material.

Example: a spring with a weight attached to one end.! Compressive stress that tends to compress a material. Example: a

car jack under load.

! Shear stress that results when a transverse load is applied. Example:to a shaft that is misaligned.


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Notes:The three types are illustrated in Figure 1.

TENSION COMPRESSION SHEAR

Figure 1

Types of Stress

Strain is the resulting effect of stress. For example, strain is often

measured as the % of elongation or relative change in the length of a

part to which a tensile force has been applied. In many materials,applied stress can have one or more of three effects, depending on the

magnitude of the stress:

! At relatively low stress, material will deform elastically, that is, it

will return to its original condition after the stress is removed.

! At moderate levels of stress, the material will reach its elastic limitor yield point and begin to deform plastically. It will not return to its

original condition, but will remain permanently deformed.

! At relatively high levels of stress, the material will fail or fracture because its yield strength has been exceeded.

2.2.2 Thermal Stress and Strain

Materials are also subject to stresses because of temperature differences.

These can cause changes in the dimensions of a part. Whenever achange in dimension is resisted, a thermal stress will result. Changes in

the dimension of a unit can be resisted by external constraints or caused

by internal temperature gradients, or both. For example, an externalconstraint could be a beam set in concrete at both ends. When the beam

is heated or cooled significantly, compressive or tensile stresses will

result.


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Notes:In cases where a temperature difference or gradient occurs within the

material, the forces generated will be internal.

The hotter parts of the material will usually have expanded more than

the colder parts, causing compression of the hotter areas and tension in

the colder parts.

Sometimes, as opposed to the constrained case described above, this is

considered to be a true heat stress. Some of the processes involved indiscussing heat stress are described below:

! Thermal Expansion, (or the increase in length) for most materials,

results from an increase in temperature. The extent of the expansiondepends on the temperature change, the temperature itself, the length

of the part, and the material(s) involved.

! Differential thermal expansion results whenever there is atemperature difference or gradient from point to point in metals. The

differential occurs because most metals expand with increasing

temperature. If the increase (or decrease) in temperature is differentin different sections of a material, the sections will have expanded to

a different extent. In this case, the compressive and tensile stresses

can result in the bending of the part, as shown in Figure 2.

Compression

Tension Tem

p

e

r a

tu

r e

Figure 2

Bending Due to Differential Thermal Expansion

! Thermal shock is an internal stress that is the result of rapid heatingor cooling of a part. In the case of heating, high compressive

stresses near the surface can result in internal fracturing while in thecase of cooling, the high tensile stresses on the surface of the part

can result in external cracking.


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Notes:2.2.3 Hoop Stress

Hoop stress is stress in a pipe that resists the expansion of the pipe whenthe fluid inside the pipe is under pressure. In more technical terms it is

the stress in a pipe wall acting circumferentially in a plane perpendicular

to the longitudinal axis of the pipe and produced by the pressure of the

fluid in the pipe.

2.2.4 Residual Stress

During manufacture, installation and maintenance material may be leftin a state where there are permanent stresses inside the material.

Residual stress is applied continually for the life of the equipment.

Drilling holes, welding and bending pieces are among the many ways ofapplying residual stress onto material. The more formal definition

follows. Residual stress is a tension or compression, which exists in the

bulk of a material without application of an external load (applied force,displacement of thermal gradient).

2.2.5 Factors that Cause Mechanical or Thermal Stress in a

Component

As mentioned, most materials being stressed have a tendency to deform

reversibly (elastic deformation) at relatively low levels of stress while

those that deform permanently (plastic deformation) do so at higherlevels past the elastic limit. These concepts and others will be discussed

later and are illustrated in Figure 3 below.

With increasing stress (force/area), the material deforms, and is subject

to increasing strain, until the material reaches its elastic limit. Any

further increase in applied stress will result in plastic or irreversibledeformation. At some point, the material will begin to fracture and then

the stress actually decreases.


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Notes:Figure 3 illustrates the effect of applying a mechanical tensile stress, but

it should be noted that materials respond in similar ways to other modesof applied stress.

region ofplasticdeformation

STRAIN

S T R E S S

e l a s t i c l i m i t

A

fracture

B

Figure 3

Plastic Deformation Regions

All materials on the earth’s surface are subject to the mechanical stresses

caused by the force of gravity. Gravity is also responsible forcompressive loads on machine supports and for potential creep in large

horizontal shafts (to be discussed below).

Other mechanical stresses occur in rotating machinery due to centrifugal

forces and loads on thrust bearings.

Thermal stresses are caused by heating or cooling as temperaturedifferentials are set up due to the flow of heat into or out of a part.

As discussed, the dimensions of a part depend on, and usually increasewith, rising temperature. The consequence is that the surface of a part

being heated expands more than the interior, causing compressive

stresses near the surface and tensile and shear stresses inside the part.While heat exchanger tubes are also subject to thermal stresses, the

effects are small because the temperature drop across a heat exchanger

tube is usually very small.

Both thermal and mechanical stresses can result from sudden changes in

pressure in a fluid (gas and/or liquid) system. Pressure is a measure ofthe force exerted by a fluid constrained by a solid surface (for example,

boiler feed water or steam in a pipe).


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Notes:Inside a pipe that is exposed to the atmosphere, the gauge pressure is a

measure of the tensile forces exerted by the fluid on the pipe.

A sudden drop in pressure may indicate the possibility of a potentially

catastrophic pipeline rupture. Should the flow of a fluid stop suddenly

for any reason, there will be a nearly instantaneous increase in thesystem pressure and steam temperature, which can cause failure due to

mechanical and thermal stresses.

2.2.6 Consequences of Exceeding Stress Limits of Materials

As materials are subject to increasing stress, the strain also increasesthrough stages where the effect is reversible, irreversible and finally,

total failure due to fracturing occurs. Clearly, the useful functioning of a

part may end long before actual fracturing because the plastic

deformation is irreversible and, therefore, the part will not return to itsinitial state.

With experience, designers have learned the relationships among stress,strain, temperature and operating conditions. With these data, stress

limits have been set that can permit operation and use of a component

for an acceptable time. Exceeding the stress limit however, can cause premature component failure, with all the associated consequences.

2.2.7 Why Heating and Cool Down Rates are Limited

Thermal stresses are generated whenever temperature gradients exist in

a material and temperature gradients are created whenever a component

is heated or cooled.

As noted, due to differential rates of thermal expansion, very largegradients can result whenever a large component is heated or cooled

rapidly. To avoid the possibility of internal or external failure of the

component, heating and cooling down rates are designed to limittemperature differences and the consequent strains due to thermal

stresses to acceptable levels.


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Notes:2.2.8 Why valves should be cracked open during warm-up of a

system

During warm-up of a system, large temperature differences may develop

on either side of a closed valve. The thermal stresses that develop across

the valve can cause valve-failure by the mechanisms previously

discussed.If valves are not cracked open during a system warm-up, the expanded

volume of the confined water in the pipes, tanks and other vessels,

which comprise the solid containment, will cause an increase in pressureand temperature. The resulting additional mechanical stress or opening

of pressure relief valves, are undesirable consequences.

2.3 BRITTLE/DUCTILE TRANSITION

2.3.1 Ductility

Ductility is a measure of the property of a material that allows it to

deform or change shape plastically under shear or tensile stresseswithout fracturing. Plastic deformation is irreversible. This means thatsome fundamental structural changes have occurred during the

deformation. For most materials, ductility increases with temperature.

Hardness, or the resistance of a material to penetration by a sharp object,

is a property closely associated with ductility. Ductile materials are

usually relatively soft, and hard materials are often quite brittle (lackingin ductility). Diamonds and alundum are examples of very hard

materials, and talc of a very soft material.

Copper for instance is a relatively ductile, hence soft material. It can be bent many times without fracturing but, as we know from experience, ifit is bent too many times, it becomes work-hardened, then brittle and

fractures easily.

Toughness is a related property that combines ductility and strength.Leather is an excellent example of a tough material. Generally, ductile

metals will also tend to be tough.

2.3.2 Brittleness

Brittle metals have relatively little capacity for plastic deformation

before they fracture. This means that a brittle part under excess stressloading can fail suddenly and potentially catastrophically without any

initial elongation or deformation. The less ductile a metal, the more

brittle it will be.


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Notes:2.3.3 Nil-Ductility Transition Temperature

Certain metals, such as carbon steel, and some plastics experience afairly sharp change from a ductile to a brittle behaviour as the

temperature falls.

One way of characterizing this property involves measuring the energy

absorbed by the material to the point of fracture as a function oftemperature.

The nil ductility transition temperature is: the temperature at which theenergy needed to cause fracture is half the difference between those

energies required for ductile and for brittle fracture. This concept is

illustrated in Figure 4, below.

BrittleFracture

Ductile behaviour

Difficult crackinitiation and growth

Easy crackinitiationand growth

Ductile / Brittle Transition Region

Temperature

F r a c t u r e

E n e r g y

Figure 4

Nil Ductility Transition Temperature

Metals that display this property are known as notch sensitive as failure

is particularly likely to occur at zones of high stress, such as those

caused by a notch.

At temperatures below the transition temperature, fracture is brittle, with

easy crack initiation and growth.At temperatures above the transition temperature, fracture is ductile and

crack initiation and growth are difficult. At temperatures close to the

transition temperature, cracks may grow rapidly.


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Notes:2.3.4 Differences between Ductile and Brittle Fracture

The following Table 1 summarizes the differences between brittle and

ductile fracture:

Generally, ductile fracture is the preferred failure mode becauseappreciable plastic deformation occurs before fracture, giving early

warning, if noted, of impending trouble.

Characteristics of a Ductile

Fracture

Characteristics of a Brittle

Fracture

Primarily a flow process (i.e.,

deformation) where thematerial is slowly torn apart

with a large expenditure of

energy.

Involves little or no flow

(deformation) of material.

Crack growth is a slow process. Significant plasticdeformation before and

during crack growth.

The crack growth is rapid (about2000 m/s). (Brittle fracture is possible only when cracks can

propagate at high velocities.)

The crack grows through the

grains (transgranular) and the

fracture appearance is greyand fibrous.

The crack grows along the grain

boundaries (intergranular) and

gives the fracture surface a brightcrystalline or granular

appearance.

Table 1

Characteristics of Ductile and Brittle Fracture

2.3.5 Operating Limitations for Materials Exhibiting a

Ductile/Brittle Transition Temperature

Clearly, the safety and integrity of the facility depend on operating with

all materials well above the nil ductility transition temperature.

When this is impossible, as during the use of isolation ice plugs, the

temperature of the piping in the area of the plug will be well below the

transition temperature of carbon steel.

Every precaution possible must then be taken to avoid mechanical and

thermal shocks in the area of the plug. The same precautions must applywhenever work, for any reason, is done on materials below the transitiontemperature.


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Notes:The turbine and generator shaft have transition temperatures in the range

90-1000C. To avoid brittle fracture due to thermal stresses, turbine and

generator shafts are pre-warmed initially with the shaft rotating slowly,

which evens out the stresses.

2.4 CREEPCreep is the permanent or irreversible, time-dependent deformation of a

material under a continuing loads. If the deformation is not managed properly, cracking and permanent failure can result. Examples of

conditions under which metals must sustain a continuing load include:

! A stationary turbine shaft supporting its own weight (gravitational

force)

! Walls of vessels or piping e.g., pressure tubes in a CANDU station,operated under pressure (pressure force)

! The blades on a turbine rotor (centrifugal force)

! Cables in pre-stressed concrete beams (imposed tensile force)

! Overhead power lines (gravitational force)

For a given material, the creep rate will increase under the followingconditions:

! An increase in applied stress

! An increase in temperature

! An increase in neutron fluxes (where applicable) combined withelevated operating temperatures.

Normally, neutron irradiation damage tends to harden a material but, atelevated temperatures, the increase in ductility with temperature actually

more than offsets the normal hardening process in a neutron field.

2.4.1 Why a large shaft becomes deformed when at rest

The force of gravity applies a constant load to turbine shafts, whether at

rest or rotating. This stress always applies downward towards the centreof the earth. When the rotor is stationary, the metal is stressed in one

direction only and tends to sag. Initially, the turbine shaft experiences

time dependent-anelastic stress.Ultimately, the sag may become irreversible, and the resulting creep can

lead to extra stress on bearings, vibration, and potential failure of the

turbine.


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Notes:2.4.2 Why rolling a large shaft prior to operation reduces the

deformation

Rolling the shaft evens out the effect of the gravitational stress on the

shaft metal and reduces or eliminates the effects of anelastic stress and

sag. If the turbine is left stationary for a long time, the undistributed

stress may result in creep, which is irreversible. For this reason, turbinesmust be rolled regularly.

2.5 FATIGUE

2.5.1 Failure mechanisms due to fatigue caused by work

hardening

Fatigue is a failure phenomenon associated with work-hardening of

materials caused by fluctuating or repeated loads that result in increased brittleness and reduced service life. Fatigue is characteristic of ductile

materials but the final failure is rapid and characteristic of brittle

fracture.The time leading up to fatigue failure cannot be predicted exactly but thefollowing conditions are known to result in work- hardening, leading to

fatigue:

! a relatively large, fluctuating applied stress

! a sufficiently large number of stress cycles

Fracture failure due to fatigue often occurs at relatively low applied

stress and well within specified design loads. The stress can bemechanical, thermal or both and can alternate between compression and

tension, or simply alternate between high and low values.

The best ways to ensure a reasonable service life for parts subject to theabove conditions is; to ensure that scratches and other surface

imperfections are removed by polishing and, if possible, by heat

treatment, which relieves internal strains.

The only reasonably reliable methods of checking the progress of fatigue

are by visual, x-ray or ultra-sound examination for surface and othercracks of the parts likely to be subject to work- hardening.

2.6 EROSION

2.6.1 Surface failure due to wear and erosion of materials

Surface failure is the loss of material from components due to corrosion

or wear. The main causes and mechanically based mechanisms of wear

are discussed in this section.


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Notes:The main causes of wear failure are abrasion, adhesion and fretting.

Wear failure would be much less of a concern if solid surfaces werecompletely smooth. Unfortunately, even highly polished surfaces have

microscopic imperfections, with high spots, or hills and low spots

(valleys). The load on a part is not uniformly distributed but tends to be

concentrated on the hills. This means that stress levels on the hills can be severe and beyond design limits

.

The three mechanisms of abrasion, adhesion and fretting are illustratedin Figure 5 below. All three mechanisms depend on the motion of

surfaces relative to each other.

ABRASION

ADHESION

FRETTING

Figure 5

Types of Surface Wear

2.6.2 Abrasion

Abrasion is the microscopic gouging of a softer surface by the hills of a

harder moving surface, or by hard particles trapped between the moving

surfaces. The particles may be brought into the system from outside ormay be the broken bits of hills from another surface.

Filtering out the hard particles from a re-circulating lubricant reduces the

effects of abrasion on seals and glands.


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Notes:Moving parts in slurry and gas/liquid transport are also subject toabrasion. Specially designed seals and regular maintenance are

important to minimize abrasion in these systems.

Cavitation also results in the erosion of valves and pumps. Very highstresses are caused as vapour bubbles collapse, initially causing

embrittlement that will finally lead to the development of pits and

cavities.

2.6.3 Adhesion

Adhesion is a process of surface failure in which the hills of metal parts

are fused or welded together on a microscopic level, followed by the

ripping off of the welds as the parts move relative to each other.

Relatively rough and soft surfaces are particularly prone to this type of

failure.

To avoid this type of failure, run-in periods are specified for new

machines and parts - high loads are then avoided and adequate

lubrication is provided to ensure the controlled reduction of surfaceroughness.

2.6.4 Fretting

Fretting is usually associated with corrosion and occurs when corrosion

products, such as oxides, accumulate between two surfaces in small

displacement, periodic motion relative to each other. The oxides are

quite hard and exacerbate the wear process.

This type of surface erosion occurs frequently in piping and heat

exchangers, where even a small vibration can start the fretting process.


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Notes:REVIEW QUESTIONS

MECHANICAL & THERMAL STRESS

1. Define the following terms as they relate to materials.! Stress

! Strain

! Thermal expansion

! Differential thermal expansion

! Hoop stress

! Residual stress

2. Give four examples of mechanical forces that put stress on

equipment in a power plant.

3. A tank containing a gas undergoes a sudden rapid pressure drop.This pressure drop causes thermal stress in the walls of the tank.Briefly explain this phenomenon.

4. There are operating limits as to how faster equipment can beheated or cooled during plant start-ups and shutdowns. Explain

the potential consequences of exceeding theses limits.

5. Define the following terms.

! Ductility

! Brittleness

! Nil-ductility transition

6. Fractures of materials can be divided into two major types:

ductile and brittle. Describe the difference between these twotypes of fracture.

7. A material exhibiting a ductile/brittle transition temperature will

have operating limitations. State 2 typical limitations and explainhow they protect the material.

8. Define the term creep as it relates to materials.

9. Explain why the rotor of a turbine generator is occasionallyrotated even during a shutdown.


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Notes:10. Before start-up of a large turbine generator after a shutdown it is

placed on turning gear for a number of hours. Explain why this isa standard operating practice.

11. Describe how metal components fail by the mechanism referred

to as fatigue.

12. There are three major mechanisms that cause surface failure.

Name the three methods and briefly describe each.


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Notes:3.0 CORROSION

3.1 INTRODUCTION

Almost all metals except noble metals like gold, silver and, rarely, copperare found in nature in chemical combination with non-metals, that is, they

have transferred to or shared electrons with other elements. The minerals

so formed through chemical combination are often different from eachother, but under the geological conditions in which they are found, they are

in a stable and preferred state.

Corrosion is the undesirable combination of processes by which metals

tend to chemically bind with other materials by losing or sharing electrons

to or with other elements.

The chemical tendency for metals to form compounds by chemical reactionis not always undesirable. For example, as will be discussed below, carbon(mild) steel can react with oxygen to form the mineral magnetite, which

protects the steel from corrosion.

3.2 TYPES OF CORROSION

The common types of corrosion are reviewed below.

3.2.1 Uniform Corrosion

Uniform corrosion is characterized by a relatively even rate of corrosion

over an entire exposed surface. This type of corrosion is usually

anticipated for parts like structural elements that can be sacrificed overtime, but affects all common metals. In the case of carbon steel, the iron

reacts with oxygen to form the mineral magnetite, an iron oxide, with theformula Fe3O4.

Magnetite forms a protective layer that decreases the rate of corrosion. Themagnetite layer consists of tiny, tightly packed black crystals that are dense

and impermeable.

The formation of this protective layer depends on pH. At low pH (highacidity), no protective layer forms, and the steel will dissolve rapidly. At

high pH, > 12, the magnetite layer is no longer stable, and relatively porousrust permits rapid corrosion. Rust is another hydrated form of iron oxidethat is porous and tends to be non-crystalline (amorphous) and has the

approximate formula Fe(OH)3·xH20.


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Notes:The effect of pH on uniform and other types of corrosion on somecommon materials is discussed below:

! As noted above, the corrosion rate of carbon steel is a minimum in the

pH range of 10 – 12.! Many common metals, like steel, aluminium and magnesium are

subject to acid attack at low pH.

! Zirconium alloys have high corrosion resistance to acidic solutions andalkaline solutions up to pH 12.

! A near neutral pH is best for stainless steel and copper alloys, and isabsolutely essential for aluminium.

! Nickel alloys such as monel and inconel demand a high pH

environment.

The addition of chemicals and the use of surface coatings (paint) usually

control the rate of uniform corrosion.

3.2.2 Galvanic Corrosion

Figure 1A

Typical Galvanic Cell

Each metal or alloy has a different tendency or potential for donating or

sharing electrons. If two dissimilar metals say copper and carbon steel areconnected in an electric circuit (galvanic cell), as shown in Figure 1, the steel

will tend to donate electrons to the hydrogen ions in the aqueous solution, producing hydrogen gas (H2). The aqueous salt solution (electrolyte) acts both as a conductor of electricity, and as a reagent.


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Notes:The galvanic cell shown has two electrodes; the steel anode, which donates

electrons, and the copper cathode, which receives them and transfers themto hydrogen ions.

The galvanic cell can also be used to help prevent corrosion. For example,

if the copper electrode in the scheme above is replaced with magnesiummetal, the steel electrode will become the cathode as the magnesium

electrode (anode) dissolves (corrodes) preferentially to produce positive

magnesium ions. The magnesium metal dissolves preferentially because ithas a much greater tendency than steel to donate electrons. This procedure

is called cathodic protection. Whenever two different metal electrodes are

connected as shown in Figure 1, the metal with the greater tendency todonate electrons will always form the anode, and the metal with the lower

tendency will always form the cathode. Other factors, which affect the rate

of galvanic corrosion, include:

!

The oxygen concentration of the electrolyte (process water). Oxygen can pick up some of the electrons from the cathode, and react with hydrogen

ions to produce water.

! Increases in temperature, generally, increase the rate of chemicalreactions. Corrosion is a chemical reaction.

! The conductivity of the water (electrolyte). Clearly, the waterconcentration should be as low as reasonably achievable (ALARA) to

increase the resistance of the circuit to the transfer of electrons and thereby

reduce the corrosion rate.

! The ratio of the surface area of the cathode to that of the anode(cathode/anode surface area ratio). The corrosion rate increases as the

relative cathode area increases because the resistance to electron transferdecreases (current flow increases).

Galvanic corrosion occurs whenever two dissimilar metals are in electrical

contact with each other in water that can transfer electrons, that is, has non-zero conductivity.

Galvanic corrosion, then, can be minimized by selecting metal couples that

have relatively close potentials to donate electrons, by isolating dissimilarmetals from each other, and also by isolating the anodic metal from the

process water. An example of how to isolate dissimilar metals form each

other is the use of a plastic isolating bushing in pipe couplings connectingsteel and copper. The protective magnetite layer formed on steel in the right

pH range tends to isolate the anodic metal from the electrolyte (process

water).


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Notes:3.2.3 Pitting and Crevice Corrosion

Pitting and crevice corrosion are special cases of galvanic corrosion bywhich metal loss is localized over a relatively small region of a metal.

In these cases, corrosion occurs because of localized concentration

differences of oxygen or of a hostile ion, for example, the chloride ion, or

of differences in pH. This situation can develop wherever conditions in theflow result in regions of low flow or stagnation, for example, in small

cracks, gaps, or crevices, such as those associated with bolts, gaskets, and

metal contact points.

Pitting corrosion is associated with scale and crud deposits which form a

barrier between the main flow and the stagnant water trapped under thescale or crud deposit. Under these conditions, oxygen dissolved in the

process water cannot penetrate the scale. In the relative absence of oxygen,

the metal under the deposit becomes anodic, that is, it will tend to dissolve

and form a deep pit. This situation is illustrated in Figure 2.

Scale

Region where pit will form

Metal

Electrolyte (H2O plus dissolved oxygen)

Figure 2

Pitting Corrosion

Pitting and crevice corrosion are minimized by chemical control, properdrainage of vessels during shutdown, removal of suspended solids by

filtration, removal of corrosion products by ion exchange, and removal of

scale, crud and bacterial deposits.

Oxygen control and removal is important, because this tends to reduce the

chemical potential for the formation of oxygen depleted, anodic zones.


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Notes:3.2.4 Stress Corrosion Cracking (SCC)

Stress corrosion cracking can occur whenever steady tensile stressescombine with corrosion to cause the formation of localized cracks.

Typically, SCC is found in stainless steel systems and is promoted

aggressively by chloride ions. SCC can cause sudden failure without

warning.

The tensile stresses are caused during part manufacture, from faulty

installation, or from in-service conditions.

Methods of controlling SCC are discussed in section 3.4.

3.2.5 Erosion Corrosion

Erosion corrosion occurs when the flow of water combines with corrosion

to effect significant increases in the rate of metal removal over thatexperienced at lower flow rates. The effect of the flow is due to the

removal of metal ions and protective layers as they form, which increasesthe chemical driving force for corrosion and may tend to prevent theformation of a protective layer. This latter effect is a major cause of loss of

metal in copper alloy steam condenser tubes. Corrosion product fines and

other particulates like sand and silt in the cooling water can also increase

corrosion rates.

Erosion corrosion has also been observed to cause thinning of carbon steel

outlet feeder piping in the primary heat transfer system (PHTS). Thecontrol of this phenomenon is discussed in the sections below.

Grooves, waves and valleys in the metal surface, and short time periods tounexpected failures characterize erosion corrosion. Erosion corrosion is

promoted by high water velocity, turbulence, the presence of particulatesand the impingement of water at high velocity on metal surfaces. These

effects are minimized by good design.

3.2.6 Microbiologically Induced Corrosion (MIC)

Bacteria, commonly in stagnant water promote microbiologically induced

corrosion (MIC). The process can take place with or without the presenceof oxygen, because specific bacteria have evolved to take advantage of

either condition. For example, thiobacillus ferroxidans can oxidize sulphur

to sulphuric acid in water-cooling systems. Nodules of oxygen loving bacteria can deposit on metal surfaces, creating

conditions similar to those described in the section on Pitting Corrosion.

Chlorination and replacement of exchanger tubes are used to control

corrosion in heat exchangers using service water.


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Notes:The elimination of stagnant conditions is a major factor in the minimization

of MIC in systems such as irradiated fuel bays, or demineralized waterstorage tanks. In systems where water is being circulated, e.g., through

heat exchangers, a pH > 10.5, and the exclusion of air help minimize

bacteria growth. Bactericides, chemical treatment and filtration are also

used for control.

3.3 CARBON STEEL BASED SYSTEMS

The PHT, steam and feedwater, endshield cooling and condensate systems

are generally considered to be the carbon steel systems.

3.3.1 pH Control in Carbon Steel Systems

As noted in the section above, pH is generally maintained greater than 10

and less than 12 to minimize the corrosion rate of carbon steel. The rangein the PHT is 10.1 to 10.4. The pH on the Secondary side is generally 9 to

9.8 at condensate feedwater conditions (Pickering is lower). The pH may

approach 12 at boiler temperatures.The relatively narrow range of pH is necessary to minimize the rate of thedissolution of the protective magnetite layer, to prevent the acid attack of

the steel at low pH, and to reduce the rate of conversion of magnetite to

porous rust by dissolved oxygen at high pH.

Figure 3

Solubility of Magnetite as a Function of Temperature at Different pH


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Notes:The Primary Heat Transfer System (PHTS)

At a high pH, say 10.2.as illustrated in Figure 3 above, the solubility ofmagnetite is quite high and decreases with temperature. The consequence

of this is that magnetite will tend to dissolve near the reactor outlet header

where the temperature is highest, and deposit on cooler parts of the PHTS,

endangering the protective zirconia layer. Corrosion of the pressure tubes beneath the fuel bearing pads will also be promoted. Damage to the

zirconia layer can result in increased hydriding of the pressure tubes.

Lithium deuteroxide can concentrate at the fuel bearing pads, and attack thezirconium alloy by the mechanism of crevice corrosion. Thinning of the

outlet feeder pipes due to erosion corrosion is also a concern when the pH

is off spec on the high side.

High pH on start-up is common in units where lithium deuteroxide hide out

has occurred. When cooling down or lowering reactor power, the HTSshrinks, drawing heavy water and concentrated lithium deuteroxide from

the pressurizer into the HTS, causing a high pH excursion in the HTS.Valving in a deuterated IX column to replace excess lithium with

deuterium ions controls high pH.

At a low pH, say 9.3 as shown in Figure 3, magnetite is less soluble at

higher temperatures, and will deposit on the hotter in-core surfaces, leadingto reduced heat transfer, fuel sheath hot spots, and fuel failure, (Increased

corrosion of carbon steel will also cause increased activated corrosion

products in the core, and higher fields out-of-core.)

One or more of the following causes low pH:

!

Spent IX columns.! Presence of lithium as carbonate rather than deuteroxide. Carbonate is

a much weaker base than deuteroxide: Carbonate can come in the resinas purchased, by the radiolysis of organic matter, or from air if the HTS

is open during shut down.

! Lithium deuteroxide hide out in stations with passing pressurizer

valves. Steam escapes from the pressurizer to the bleed condenser andis replaced by HTS water, containing lithium deuteroxide. As the

steam escapes, the concentration of the non-volatile deuteroxide

increases in the pressurizer. A corresponding drop in system pH willoccur requiring the addition of lithium deuteroxide. (There are no

pressurizers at Pickering.)

Low pH is controlled by addition of lithium hydroxide and/or by valving in

fresh IX columns, as required.


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Notes:The Condensate, Feedwater and Boiler System (C, FW, B System)

CANDU stations in Ontario use volatile bases (morpholine and/orammonia, usually by hydrazine addition) for their pH control to minimize

corrosion in ferrous systems. For the all-volatile treatment (AVT),

volatility is important because all water cycling through the boiler is

vaporized. If the added chemicals did not boil off with the water, theywould tend to concentrate in sludge deposits in the boiler, creating

corrosive conditions.

The chemicals used to control pH and minimize corrosion are added

downstream from the condensate extraction pump (CEP).

The condenser and feedheater tubes of C, FW, B systems in all newer

CANDU stations are stainless steel, the piping and most other components

are carbon steel. Thus, the C, FW, B systems are essentially all-ferrous.

In all-ferrous stations, on–line pH measurements are made ahead of the LPfeed heaters, to ensure control of condensate pH in the range 9.8 to 10.0.

Ammonia maintains control of pH in most stations with all-ferrous trains.The ammonia forms as a result of the decomposition of excess hydrazine,

or may be added directly. Morpholine is also specified in all-ferrous

stations because its use reduces corrosion product transport in thefeedwater, and also reduces erosion/corrosion in two-phase (wet steam)

regions. When the desired condensate pH is maintained, the boiler water

pH will follow at the desired value of > 9.5.


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Notes:Auxiliary Systems

Auxiliary systems, although not covered in the Objectives or K-Statements

of the Module, also require pH control. Auxiliary systems include End

Shield Cooling, Liquid Zone Control, Recirculating Cooling Water, and

Irradiated Fuel Bays. Control is affected by ion exchange, as noted inTable 1 below:

System Resins in

Mixed Bed

Purpose

End Shield

Cooling

Li+, OH

!- To remove unwanted impurities

(ions) and to maintain system pH(typically pH 10.0 – 10.5)

Liquid ZoneControl

H+, OH


(ions) and to maintain system pH

(typically conductivity < 0.1 mS/m,

pH 6.0 – 8.0)Recirculat-

ing Cooling

Water

Li+, OH


(ions) and to maintain system pH

(typically pH 9.0 – 10.8)

Irradiated

Fuel Bay

H+, OH


(ions) and to maintain system pH(typically pH 6.5 – 7.5, conductivity

< 0.2 mS/m)

Stator

Cooling

Water

H+, OH

!- and

Deoxygen-

ating

Column

To remove dissolved oxygen by

deoxygenating column and other

unwanted impurities (ions) in a

mixed bed column, and to maintainsystem pH (typically pH 6-8,

conductivity < 0.1 mS/m, dissolved

oxygen < 20 ppb)

Table 1

pH Control in Auxiliary Systems by Ion Exchange

3.3.2 The importance of dissolved oxygen control, and typical methods for

proper control

The Primary Heat Transfer System (HTS)

Elemental oxygen gas is produced in the primary heat transfer system (HTS)

by radiolysis of heavy water (D2O). Maintaining low dissolved oxygen

minimizes the corrosion of carbon steel and zirconium.


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Notes:Low oxygen gas concentration is also important because:

! Oxygen can cause stress corrosion cracking of Inconel boiler tubes and

corrosion of Monel-400.

! Oxygen gas increases the aggressiveness of the chloride ion.

These concerns are discussed in the section below.

Oxygen corrodes zirconium and carbon steel, and can convert protectiveoxide (zirconia and magnetite) into porous hydrated oxide layers. This

porosity permits water to attack the underlying metals, producing oxides

and deuterium:

2Fe + 3D2O # Fe2 O3 + 3D2

Maintaining dissolved deuterium within specification (3-`10 cc/kg D2O)

controls dissolved oxygen in the HTS. The slight excess of dissolveddeuterium (or hydrogen) consumes dissolved oxygen by the following

reaction2D2 + O2# 2 D2O.

The Condensate, Feedwater, and Boiler Systems

For convenience, the term Condensate, Feedwater and Boiler (C, FW, B)System refers to the complete secondary (light water) side. The overall

working unit includes the boiler, turbine, condenser, condensate extraction

pump, LP feedheater, de-aerator, boiler feed pumps and HP feedheaters.The governing Objective of chemical control is the minimization of

corrosion and its consequences, particularly sludge and scale formation.

Dissolved oxygen gas is the most harmful impurity in the condenser, feedwater and boiler system because, as discussed above, it converts protective

magnetite on carbon steel to porous rust, and promotes corrosion

throughout the system. Corrosion products lead to tube sheet sludge andcrud deposits. These adverse effects increase with increasing oxygen gas

concentration.

Dissolved oxygen increases the aggressiveness of the chloride ion,

particularly under localized anodic conditions that encourage pitting.

Because of its aggressive chemistry, dissolved oxygen has action levels in

the feedwater system to minimize corrosion product formation andtransport in the boilers, and to minimize corrosion, particularly pittingcorrosion of the boiler tubes in the presence of chloride.


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Notes:To maintain the dissolved oxygen As Low as Reasonably Achievable

(ALARA), dissolved oxygen control is achieved in three stages:

The first stage is the condenser air extraction system. Most dissolved

oxygen or air removal occurs above the condenser water line. Sources

below the water line are the main contributors to the dissolved oxygencontent in the condensate.

In the second stage, dissolved oxygen is removed from the condensate bythe deaerator, which contains sprays, steam spargers, and cascade trays

over which the hot water tumbles. The combination of heat and large

surface area provides very efficient stripping of dissolved gases. Oxygen isreduced to < 5 ppb (wt/wt) and non-condensable gases are also removed.

This efficient stripping reduces chemical consumption in the final stage.

In the final stage of oxygen removal, hydrazine is added to chemically react

with the remaining dissolved oxygen to produce nitrogen gas and water.

An excess of hydrazine is required to control the dissolved oxygen below 5 ppb. The excess will cause the pH to rise, as hydrazine slowly converts to

ammonia, a weak base. This compound too must be controlled to protect

against corrosion of ferrous brass systems containing copper, which isdissolved in ammonia. (In all-ferrous systems, the concern is an

environmental release in the blowdown water)

3.4 THE IMPORTANCE OF CONDUCTIVITY CONTROL, AND

TYPICAL METHODS FOR PROPER CONTROL

The Primary Heat Transport System (HTS)

Conductivity and lithium are not specifically controlled in the HTS but areimportant diagnostic parameters. They are used in trouble-shooting at all

action levels, if the HTS pH goes out of specification. If the amount of

lithium present is substantially greater than the system pH would indicate,conductivity measurements of grab samples are used to help explain this

discrepancy. The presence of other ions (such of carbonate) would usually

show from the conductivity. Correlations between conductivity and theconcentrations of ionic species are possible because the specific

conductivities of individual ions are known.

The Condensate, Feedwater and Boiler Systems

As noted above, the term Condensate, Feedwater and Boiler (C, FW, B)

System refers to the complete secondary (light water) side. The overall

working unit includes the boiler, turbine, condenser, condensate extraction pump, LP feedheater, de-aerator, boiler feed pumps and HP feedheaters.


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Notes:The governing objective of chemical control is the minimization of

corrosion and its consequences, particularly sludge and scale formation.The latter problem area is discussed in the section below.

Boiler water cation conductivity is an operating parameter with a shutdown

limit. The impurities are both aggressive inorganic anions such as sulphateand chloride, as well as more benign organic anions from the additives or

the water treatment plant. Since the organic anions are less corrosive, the

inorganic contribution must be determined before shutdown. Maintainingthe inorganic contribution to cation conductivity ALARA, protects against

boiler tube corrosion.

Boiler water has a high background conductivity from pH additives

(morpholine, hydrazine and ammonia), traces of raw cooling water salts

and dissolved corrosion products. Because of the masking conductivityeffect, ordinary (specific) conductivity is not sensitive enough to be used

as a control parameter. Cation conductivity is used because it is moresensitive to the presence of harmful anions, like chloride, sulphate, and

silica.

Possible causes of high cation conductivity include a water treatment plant

acid excursion, which allows sulphate ion to enter the makeup water.Alternatively, a leak in one or more of the condenser tubes will admit raw

water into the C, FW, and B System.

Cation conductivity is reduced and/or controlled by boiler blow-down, and

by the repair of leaks, if necessary. If the conductivity is not controlled,the consequences include:

! Boiler tube scale formation;

! Boiler tube corrosion and failure;

! Loss of the protective magnetite layer on carbon steel surfaces permitsgeneral corrosion (acid excursion only).

To determine the cation conductivity, a sample of boiler water is passedthrough a cation exchange column. The column replaces all cations

(including those of morpholine, ammonia and hydrazine) with hydrogen

ions.The water exiting the cation exchange column will contain hydrogen ions

and various anions. By doing this, the conductivity signal is heightened because the anions are more likely to be fully ionized, that is, to have theirmaximum negative charge. The conductivity of water treated in this way

is called the Cation Conductivity. The desired cation conductivity for

boiler water is ALARA. The specification is < 0.25 m/S/m for most

stations.


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Notes:The use of very low conductivity makeup water from the water treatment

plant is a major factor in maintaining the conductivity of the boiler waterALARA. Continuous blowdown controls the boiler water conductivity by

constantly removing impurities from the system. Cation conductivity of

the boiler water is monitored by continuous analysis, with grab sample

verification.

3.5 STAINLESS STEEL BASED SYSTEMS

Systems covered as Stainless Steel based are the Moderator, Endshield

Cooling and the Liquid Zone Control System (LZCS)

The phenomenon of stress corrosion cracking is defined in previous

section 3.2.3

3.5.1 SCC in the Moderator

Stress corrosion promoted by chlorides can occur in the calandria and other

stainless steel parts of the moderator system. Although high chlorides are nota common problem, the most likely chloride source is spent resin in the ionexchange column. Chlorides are removed by proper ion exchange control.

3.5.2 SCC in the End-shield Cooling and the Liquid Zone Control

Systems

End Shield Cooling system materials are as follows:Pumps-stainless steel;

! Expansion tank-carbon steel;

! Filter-stainless steel;

! Ion exchange column-stainless steel;! Strainer-carbon steel;

! Valves-majority carbon steel with stainless steel relief valves;

! Piping-carbon steel;

! Heat exchanger-admiralty brass, Darlington-titanium plates.

This system has a pH specification of 10 to 10.5 with a desired value at10.2. The pH is maintained by circulation through lithiated ion

exchange resins at all stations. The other control parameter for this

system is 0.2 mg/kg. Hydrazine cannot be used in this system to controldissolved oxygen because it may promote hydrogen production and

result in a cover gas excursion.

The chemistry of the end shield cooling system, and of the Liquid ZoneSystem is essentially the same as that of the Moderator, except that light

water is used to control the neutron flux and reactivity.


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Notes:High chloride ion concentrations can promote stress corrosion cracking.

As noted, the source of chloride is a spent IX column eluting into thewater. Replacing the spent column controls it. Oxygen is also

associated with SCC, promoting the effect of chloride ion.

3.5.3 pH Control in Stainless Steel Based Systems, andTypical Methods to Maintain Proper Control

pH Control in the Moderator

Moderator pH is controlled only in conjunction with conductivity using ion

exchange columns to remove impurity ions, and replace them with

deuteroxyl and deuterium ions, that is, with heavy water. When moderatorconductivity is held at its practical lower limit, the heavy water is nearly

pure, and ionized very slightly. As the pH of the moderator moves from

neutral in either direction, the conductivity increases. The pH is analyzedon power only to troubleshoot high conductivity.

An exception occurs whenever gadolinium nitrate is present in relatively

large amounts in the moderator heavy water. In this circumstance, becausegadolinium deuteroxide may precipitate out if the pH is too high, the pH

specification is 4-6. (The pH becomes a control parameter during over-

poisoned guaranteed shutdown).

Under special circumstances, (during unit start up), when gadolinium nitrate

is present, the pH of the moderator may drift to 7 and higher. This canoccur when the cationic resin in the mixed bed IX column is spent before

the anionic resin, releasing excess deuteroxyl ions. The result is an increasein pH above 6.

Localized high pH can also occur within an ion exchange column where,after the resin has been slurried, stratification of resin beads occurs, resulting

in a high proportion of anion resin in a localized zone above the cation resin.

Elution of deuteroxyl ions from the anion resins will increase the pH of the

solution and cause the precipitation of gadolinium deuteroxide within thecolumn. To ensure sufficient cation resin is present to remove gadolinium,

and to prevent the precipitation of the gadolinium deuteroxide in the

column, stations add extra cation resin to the mixed bed to minimize the possibility of separation and to ensure the bed does not spend on cation

(Gadolinium is present as a cation).


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Notes:High moderator pH is caused by:

! Inadvertent use of the heat transport system resin (lithium based) in an

IX column.

! Too much anion resin relative to cation resin the IX columns, removing

gadolinium ions but not nitrate ions.

! Contamination of the moderator by alkaline sources such as heattransport system heavy water or lithium hydroxide.

High moderator pH is controlled by the proper use of the ion exchangesystem.

Low moderator pH is most likely due to air leakage into the cover gas. Thenitrogen in air is converted by radiolysis to nitric acid, which lowers the pH

of the moderator.

Low pH will accelerate corrosion of the heat exchangers and increase the

concentrations of deuterium gas and oxygen in the cover gas.

Low pH is controlled by:

! Minimizing air in-leakage;

! Purging cover gas when the nitrogen concentration is too high, forexample, after the system has been opened for maintenance;

! Ensuring the IX columns are operating properly.

pH Control in the End-shield Cooling and Liquid Zone Control

Systems

The chemistry of the LZCS, including its cover gas, and of the end shield

cooling system, is the same as that of the moderator system, except thatgadolinium is not used. However, conductivity is the only parameter usedfor the control of water purity in the end shield cooling and LZC systems.

Impurities, especially chloride ion, promote the release of radiolytichydrogen and oxygen gases, which, together with nitrogen and argon, are

monitored in the cover gas. As discussed above, nitrogen is convertedradiolytically to nitric acid, and argon can be activated to Ar-41, increasing

fields in the piping areas. The above contaminants in the cover gas are kept

ALARA.


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Notes:3.5.4 Conductivity Control in Stainless Steel Based Systems, and Typical

Methods to Maintain Proper Control

Conductivity Control in the Moderator

Corrosion can affect the calandria body, calandria tubes, reactivity

mechanisms, liquid zone tubes, cleanup circuits, heat exchangers and anyother metal surfaces exposed to the moderator.

Moderator conductivity is maintained ALARA to ensure that the moderatorwater is as pure as possible. As noted above, moderator conductivity is the

only control parameter for the moderator. High conductivity in the

moderator increases the rate of radiolysis, the break down of water into itscomponent parts O2 and D2 by gamma radiation. A high concentration of D2

in the cover gas gives the danger of explosive mixtures in the cover gas.

When boron is present as a reactivity poison, it has little effect on

conductivity or on radiolysis products.

Gadolinium nitrate, on the other hand, increases the formation of radiolytic

deuterium gas and affects the conductivity.

High moderator conductivity is caused by:

! Air leakage, producing nitric acid by radiolysis;

! Spent IX columns, allowing impurity ions to pass into the column effluent;

! Addition of gadolinium nitrate for reactivity control;

! Other impurities from dissolved corrosion products.

High moderator conductivity is controlled by:

! Preventing air in-leakage and ingress of contaminants;! Purging cover gas nitrogen, when high, for example, after opening the

system;

! Ensuring the IX columns are not spent.

Although controlling conductivity ALARA helps reduce corrosion rates, itsmost important objective is reducing the probability of a cover gas deuterium

excursion, which can occur very quickly, in minutes.


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Notes:Boiler Tube

HT Fluid

Scale

Flakes Boiler Water

Deposits/Sludge Pile

Corrosion

occurs hereBoiler Tubesheet or

Tube Support Plate

Figure 4

Boiler Tube Scale and Tube Sheet Deposits

3.6 SCALE FORMATION

3.6.1 Mechanisms for Formation and Adverse Consequences of Scale

Formation on Boiler Tubes-Methods Used to Minimize Scale

Over time, unless controlled, the concentration of impurities, except some

of the water treatment chemicals, will tend to build up in the boiler water.

Even with IX control, where the flow is low or restricted, localizedimpurities, for example, in crevices, can reach very high levels. The result

is the formation of sludge and sites for scale formation. These trapped

impurities are called are called hideout impurities because they are notmeasured during sampling.

Figure 4 shows boiler scale, which is a layer of foreign material built upon the surface of the boiler tubes. Suspended particulate matter is formedfrom corrosion products, and tends to settle out in regions of low flow,

forming sludge deposits. Scale can also flake off the tubes, contributing to

the formation of sludge.

To summarize, sludge and scale forming substances are:

! Suspended Inorganic Solids (Particulates);

! Dissolved Inorganic Matter;

! Dissolved and Suspended Organic Matter.

If the water treatment plant malfunctions, it too can contribute to highconcentrations of ionic impurities such as chloride, sulphate, calcium and

magnesium, in the boiler.


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Notes:Corrosion products, typically iron and copper oxides, formed throughout

the boiler steam and feed-water circuit are the main source of suspendedinorganic particulates. Smaller amount enter the circuit in the freshwater

make-up, although this water is usually very pure. The other source is raw

water entering through perforated condenser tubes, which contributes

mainly silicates of calcium and magnesium.

Some organic material can result from the breakdown of water treatment

chemicals, leakage of lubricating oil, raw water entering through failedcondenser tubes, bacterial growth in the demineralized water system, and

resin fines from the IX systems.

To summarize, boiler tube scale and sludge deposits are caused by the

presence of inorganic salts, small amounts of organic material and of

corrosion products in the boiler water.

3.6.2 Adverse Effects of Sludge and Scale Formation

If sludge and scale deposits form in sensitive parts of the boiler system,serious corrosion problems can develop under the deposit. The

concentration of impurities in low flow regions of the boiler can increase

by 10,000 to 100,000 times that in the bulk flow due to the concentratingeffect of boiling in sensitive parts of the boiler.

Pitting corrosion under sludge deposits can result in part from high

concentrations of chloride and/or sulphate at the base of the boiler tubes.Caustic soda (sodium hydroxide can cause a similar caustic attack on the

boiler tubes.

Particles entrained in the flowing water can contribute to erosion corrosion

and may settle out and form deposits of sludge that are extremely hard toremove.

Any corrosion process can shorten boiler tube life, contributing to the possibility of heavy water and tritium leakage into the boiler system.

Scale deposits on boiler tubes can also contribute to corrosion, and, inaddition, reduce heat transfer efficiency, forcing up the temperature of the

heat transport heavy water system and/or reducing heat transfer rates.

Large deposits on the tube support plates of the may also cause levelcontrol problems by restricting water/steam flow up through the boiler.


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Notes:In Summary, the adverse effects of impurities on boiler performance and

materials include:

! the accumulation of solid impurities in the form of sludge creates avery hostile corrosive environment.

! pitting corrosion and or caustic attack occurs under sludge

deposits.! erosion corrosion can be caused by suspended particles in the flow.

! scale and sludge deposits on tube support plates may lead to levelcontrol problems by restricting water and steam flow.

! over time, sludge becomes extremely difficult to remove.

! reduced tube life and increased down time will result if

perforations develop due to corrosion of the tubes.

! lost production time and outages will occur due to boiler tube leaksand time taken for deposit removal.

! scale will cause increased resistance to heat transfer, leading toincreased heat transport system temperatures and/or reduced heat

transfer.


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Notes:

REVIEW QUESTIONS

CORROSION

1. Describe each of the following types of corrosion.

! Uniform

! Galvanic

! Pitting

! Stress corrosion cracking

! Erosion

! Microbiologically induced

2. State the typical pH range that is considered optimum for carbon based steel piping systems. Explain the consequences of

operating with a pH outside of this range.

3. Describe how the pH is controlled in the heat transport system.

4. Explain the consequences of high O2 levels in water systems.

5. State the method used to control the O2 levels in the heat transport

system.

6. State three methods used to control the O2 level in the condensate and

feedwater systems.

7. Describe the negative consequences of high conductivity in the

moderator system.

8. Describe four potential sources of high conductivity in the moderator

system.

9. Describe three methods of maintaining a low conductivity in the

moderator system.

10. Define the terms sludge and scale and state three substances that

contribute to their formation.

11. State and explain four adverse consequences of excessive sludge and

scale formation in a boiler


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Notes:

Notes:4.0 PRESSURE TUBES AND FUEL BUNDLES

4.1 INTRODUCTION

Materials in nuclear reactors are exposed to ionizing radiation, for

example, gamma, beta, and alpha, as well as to fast and thermalneutrons. These forms of radiation are highly energetic, and cause

damage by interaction with the nuclei and/or orbital electrons of atoms

that constitute materials of all kinds

The changes in the atomic structure of the elements can and do causefundamental changes in the mechanical properties of materials. In

particular, ionizing gamma radiation, unlike alpha and beta radiation,

has a very high potential for permanently damaging materials.

Fast neutrons cause structural damage by displacing atoms, creating

vacancies and interstitial atoms in the crystal structure of materials.This phenomenon accounts for increased creep rates in pressure tubes.Thermal neutrons may be captured by nuclei, changing the material’s

identity. For example, cobalt-59 becomes activated to radioactive

cobalt-60. This phenomenon is one of the main reasons that theconcentration of dissolved metal ions is kept as low as possible.

Thermal neutrons also form gases, such as krypton, xenon and iodine,which may migrate to form gas bubbles that may cause embrittlement

and blisters in the fuel sheath, increasing the potential for rupture.

4.2 EFFECTS OF RADIATION ON COMMON MATERIALS

4.2.1 Oils and Greases

Oils and greases are affected both by ionizing radiation and neutrons.

Oils become more viscous (stiffer, more resistant to flow). Some oils

actually solidify, forming tar-like polymers. These phenomena reduce orstop fluid flow to some degree.

Soap-oil based greases, on the other hand, become less viscous, andmore fluid because radiation breaks down the soap-base binder.

Radiation increases the rate of oxidation by air, which also reduces the

amount of radiation required to cause significant damage.

Radiation-promoted oxidation and other damage can be limited by using

grease manufactured with special additives.


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Notes:

Notes:Both lubricants used in the fuelling machines are changed more

frequently than those used in other equipment not exposed to radiation.

4.2.2 Plastics

Plastics are formulated from long chains of molecules (polymers) and

other chemicals that control some of their properties. The chemical bonds of plastics are very sensitive to radiation, which can dramatically

change their properties. The types of chemical and structural changesthat are caused by radiation include:

! Embrittlement and increased rigidity due to cross-linking of atoms between chains;

! Loss of strength softening due to fragmentation into shorter-chain

polymers. Teflon and Plexiglas are very susceptible to this process;

! Embrittlement and rigidity from the formation of new long-chain

polymers;

! Embrittlement and increased rigidity due to oxidative cross-linking;

! Gas evolution.

Problems that have been caused by radiation damage to plastics innuclear stations include:

! Damage to the insulation on power and control cables, especiallythose within the reactor vaults;

! Damage to the outer jacket on some ion chamber instrument cables,which has been so severely affected that it has become brittle,

cracked and fallen off. Insulators in the associated connectors havealso become brittle and failed;

! Decomposed terminal blocks inside junction boxes exposed to high

radiation fields;! Damage to ion exchange resins, which are polymers, based on

polystyrene. The radio nuclides removed from active water systems

deposit much of their energy within the resin bed, leading to itsdegradation.

Programs are in place to manage the effects of these problems.

4.2.3 Metals

Fast neutrons affect the mechanical properties of metals (and othermaterials) as follows:

! Ultimate tensile strength increases;! Yield strength increases sharply;

! Hardness increases;

! Ductility decreases.


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Notes:

Notes:Fast neutrons cause vacancies and displacement of atoms in the

damaged metal.These disrupt the crystal structure and cause internal stresses, which

increase the energy, required to deform the metal

If the radiation-damaged metal is annealed at about 850-1000

0

C, themechanical properties will return almost to their original values. This

occurs because the defects become mobile at high temperature, moving

to preferred locations and eliminating some of the defects. This phenomenon is not entirely a good thing, because the process promotes

creep.

4.2.4 Carbon Steel

Carbon steel materials may benefit from the increase in strength from

fast neutron irradiation, but at the cost of increased risk of brittlefracture. Two sorts of components in CANDU operation show an

increase in the ductile/brittle transition temperature when irradiated:! The impact resistance of many pressure vessels operating at ambient

temperatures is reduced. Radiation induced brittleness of pressurevessels operating at 260

0C is not corrected by annealing because the

temperature is too low.

! The ductile/brittle transition temperature of carbon steel piping in the

Heat Transport System gradually increases over their long servicelife.

4.2.5 Zirconium Alloys

Although irradiation causes the same effects on mechanical properties of

zirconium alloys as those of carbon steel, cold working the material before installation reduces the changes.

The creep rate is increased, greatly reducing the service life of pressure

tubes made of zirconium alloys.

Fuel sheathing made of zirconium alloy becomes harder and embrittled by radiation. This does not significantly reduce the life expectancy of

fuel sheathing, but reduces the number of positions to which fuel can be

shifted to avoid handling problems after the fuel bundles are removed

from the reactor

4.2.6 Concrete

Like all materials, concrete, used extensively in nuclear stations for

structural and radiation shielding applications, is susceptible to radiation


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Notes:

Notes:damage. Gamma radiation is absorbed and converted into heat.

Neutrons also give up their energy in the form of heat and activate cobaltand other metals in the rebar.

Gamma radiation and neutron bombardment affect the structural

integri

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